In the sheet body described above, the thermal expansion coefficient of the fuel electrode layer made of Ni—YSZ cermet is generally larger than the thermal expansion coefficient of the solid electrolyte layer made of YSZ. Accordingly, the fired sheet body is likely to be deformed due to the internal stress (thermal stress) caused by the difference in the thermal expansion coefficient between the layers. The sheet body may also be deformed due to the internal stress (thermal stress) that is caused by the difference in the amount of expansion and contraction between the layers upon the firing. The sheet body is generally deformed in a direction in which the side of the air electrode layer projects (a direction in which the lower side projects).
Meanwhile, an attempt has been made to form the sheet body as thinner as possible in order to attain the miniaturization of the SOFC and reduction in the internal electrical resistance. When the sheet body is formed to be extremely thin, the support body (the layer supporting the sheet body, e.g., the fuel electrode layer) in the sheet body becomes thin, with the result that the deformation of the sheet body described above can be noticeable.
In this case, various problems arise. For example, a fuel gas channel or an air channel formed at the portion opposite to the upper surface of the fuel electrode layer or the lower surface of the air electrode layer is extremely thin. Therefore, the deformed sheet body might close these channels. Even if the sheet body is deformed to such a degree that it does not close the channels, the pressure loss generated when the fluid such as air or fuel gas flows through the channels might increase due to the deformation of the sheet body.
In order to reduce the deformation (warpage) of the sheet body, it is considered that, for example, a material having a small thermal expansion coefficient is contained in the fuel electrode layer for reducing the mean thermal expansion coefficient of the fuel electrode layer, by which the thermal expansion coefficient is made close to the thermal expansion coefficient of the solid electrolyte layer. However, when the material contained in the fuel electrode layer is an insulator, a problem might newly arise that the electrical resistance in the stacking direction of the fuel electrode layer extremely increases according to the additive amount or adding manner of the insulator.